Zeus virus question?

I use both Chrome and Firefox (occasionally IE) on my computer. I keep MalwareBytes realtime protection on. Yet every couple of days I have to shut down Firefox and remove the Zeus virus. I use Adwcleaner and then run MalwareBytes again too. That shuts it down for awhile.
Does anyone know how this thing keeps coming back and how I can kill it, once and for all.

Google it